Also: when you let the game take place at the end of / or during the last big ice age, there was so much megafaune still around (when considering rideable mounts)(13.000 years before present date).
Well known examples are mammoths and wooly rhinos, but the giant armadillo-like creatures and giant sloths of South America are less famous.

Would be so cool if there were different tribes, linked to their respective environment. Marsh people, desert nomads, mountain people, temperate forests people, each with distinctive weapons, rideable animals and their own strengths and weaknesses. And did i forget to mention neanderthalers :D